1. Field of the Invention
This invention relates to a motor driver including an inverter driving an electric motor and a booster circuit provided at the DC current side of the inverter.
2. Description of the Prior Art
A motor driver has conventionally been provided for driving an on-vehicle electric motor provided in an automotive vehicle, for example, a brushless motor. In such a motor driver, an inverter converts a direct current from a car battery as a DC power supply to an alternating current, which current is supplied to a stator coil of the brushless motor. In this arrangement, a rotational speed of the motor is detected, and the inverter controls the current by pulse width modulation (PWM) so that the detected motor speed reaches a set speed value. Furthermore, a current flowing into the inverter is detected, and current limitation is executed so that the detected current does not exceed an upper limit value, whereby the device is protected against an overcurrent.
The car battery usually has an operating voltage of 12 V, which value is relatively low. In view of drive of a high output brushless motor, a booster circuit is provided between the car battery and the inverter. In this motor driver, a current flowing into the booster circuit is detected, and the current is limited so that the detected current does not exceed an upper limit value.
In the motor driver having a battery as its DC power supply, there is a possibility of an erroneous connection between positive and negative terminals of the battery, which will be referred to as "reverse connection." As a countermeasure, a diode is connected between the battery and the booster circuit for protecting the device against the reverse connection. In this arrangement, however, current usually flows into the protecting diode in a normal operation. This results in a voltage drop. Particularly when the voltage of the DC power supply such as the battery is low, the voltage drop cannot be ignored and results in a loss depending upon the product of voltage drop and current. To overcome this problem, the prior art has provided a contractor (relay) turned on upon a normal connection of the terminals, instead of the protecting diode.
However, the above-described arrangement requires two current detecting means for detecting the currents flowing into the booster circuit and the inverter for the protection against an overcurrent respectively. This results in complication of the electrical arrangement of the device. Furthermore, the contactor for protection against the reverse connection renders the motor driver large-sized. The contactor also requires a polarity detecting circuit for turning it on and off. The polarity detecting circuit further complicates the electrical arrangement of the motor driver.